1. Field of the Invention
The invention relates to the field of dynamic magnetic information storage or retrieval. More particularly, the invention relates to the field of automatic control of a recorder mechanism. In still greater particularity, the invention relates to longitudinal position determination using a servo pattern. By way of further characterization, but not by way of limitation thereto, the invention is a method for determining longitudinal position on magnetic tape by reading a servo pattern.
2. Description of the Related Art
Magnetic tape recording has been utilized for many years to record voice and data information. For information storage and retrieval, magnetic tape has proven especially reliable, cost efficient and easy to use. In an effort to make magnetic tape even more useful and cost effective, there have been attempts to store more information per given width and length of tape. This has generally been accomplished by including more data tracks on a given width of tape which, in turn, means more data is stored on a given length of tape. In many systems, multiple volumes of data are written onto a single reel or tape cartridge. This increase in data storage density requires more accurate tape--tape head longitudinal position determination. That is, as more data is written onto a single track, precise longitudinal positioning of the tape with respect to the tape head becomes more critical. By longitudinal is meant the direction along the length of the magnetic tape.
In order to increase data track accuracy, servo stripes have been employed to provide a reference point to maintain correct lateral positioning of the tape with respect to the tape read/write head. By lateral is meant the direction across the width of the tape. One or more servo stripes may be used depending upon the number of data tracks which are placed upon the tape. The sensed signal from the servo stripe is fed to a control system which moves the head and keeps the servo signal at nominal magnitude. The nominal signal occurs when the servo read gap is located in a certain position relative to the servo stripe. Referring to FIG. 1, a one-half inch wide magnetic tape 11 may contain up to 288 or more data tracks on multiple data bands 12. With such a large number of data tracks it may be desirable to include up to five or more servo stripes 13 to improve data read and write function performance. Servo stripes 13 may utilize various patterns or frequency regions to allow precise tape to tape head positioning.
Referring to FIG. 2, a portion of a conventional servo stripe 13 is shown having two frames 14 and 15. A first frequency signal 16 is written across the width of servo stripe 13. An erase frequency is written over first frequency signal 16 in a predetermined pattern such as five rectangles 17 in each of frames 14 and 15. The five rectangles 17 in each frame result in nine horizontal interfaces 18 between frequency signal 16 and erase patterns 17. The tenth edge 19 along the bottom is not utilized for these purposes. While five rectangles are shown in FIG. 2, it should be understood that more or fewer erase patterns may be used depending upon engineering design considerations. A dashed line 21 passes along one of edges 18 and through a read gap 22 in a tape read head 23. If servo pattern 13 is passed right to left over gap 22, then gap 22 will alternate between reading frequency 16 across the full width 24 of gap 22 in areas 25 and frequency 16 across one half of read gap 22 and an erase frequency from patterns 17 across the other half of width 24 in areas 26. The servo control system in the tape drive uses the ratio of full signal amplitude in field 25 to half signal amplitude in field 26 to stay on track. While allowing for tape head positioning with respect to the tape width, these prior art servo patterns do not address the longitudinal positioning of the tape. That is, it is critical for the servo to know where along the length of the tape the data volume is stored. This information is becoming more significant in modem systems where multiple volumes of data are written onto a single reel or tape cartridge. Many prior art systems use tachometers to measure reel motor revolutions and, by knowing reel dimensions and tape thickness, the longitudinal position of the tape with respect to the tape head can be estimated within some number of meters. However, with data compression and other techniques, the data streams are compressed onto increasingly shorter lengths of tape and position estimates to within meters are not sufficient. It would be desirable to be able to more accurately determine a location along the length of the tape where the desired data volume is stored.